The long range goal of this investigation is the complete analysis of protein-protein and protein-effector interactions in the allosteric E. coli enzyme, aspartate transcarbamylase. Previous work has dealt with the energetics of binding of substrate analogs and nucleoside triphosphates to the native enzyme and its subunits. The emphasis in the next grant period will be on the energetics of protein-protein interactions, and the changes in the subunit interaction energies produced by various perturbations of the structure. Our goals are (a) to define the thermodynamic basis of assembly and (b) to provide rigorous thermodynamic criteria against which models of the allosteric mechanism can be tested. While various experimental techniques will be used as needed, the emphasis will be on microcalorimetry, hydrogen exchange, potentiometry and radioactive binding studies.